Japanese Patent Laying-Open No. 2006-214390 (PTL 1) discloses a failure diagnosis device of a fuel level sensor for detecting the amount of fuel in a fuel tank. This failure diagnosis device determines that the fuel level sensor is in malfunction in the case where the fuel level detected by the fuel level sensor does not change exceeding a determination threshold value when the vehicle has run for a predetermined running distance. This determination threshold value is set according to the amount of fuel in the fuel tank. Therefore, failure diagnosis can be executed according to the output characteristics of the fuel level sensor (refer to PTL 1).